Many Combined Heat and Power (CHP) systems also known as cogeneration are installed and operating in different process industries to provide electrical power and thermal energy. One of the most common versions uses a steam turbine to output mechanical shaft power that drives an electrical generator to produce electricity. Concurrently, during this mechanical power generation, the spent heat from the steam turbine exhaust is supplied as thermal energy either directly in the form of steam or hot water or hot air for a beneficial use. The thermal energy can be used for a myriad of applications, including process heating and for cooling applications using an absorption chiller.
Tied to the above, there are two typical scenarios of operation which can lead to operational inflexibility, wastage of energy and reduction of system efficiency, and therefore need to be addressed, viz:    a. Where the electrical generator is operated standalone and not paralleled with the utility grid, the primary control will be to maintain voltage and frequency where the electrical power output is matched with the electric load. This then determines the fluctuations in the quantity of thermal output as well as any excess or deficiency in thermal output to be compensated for. Conversely, vastly fluctuating thermal loads could occur, for example, when the thermal output is used for batch heating process systems or when the process is shutdown unscheduled. In the presence of the said such fluctuating thermal loads, a need then arises to regulate the heat supply to the thermal system and this is taken care of by installing a secondary control. If a deficiency in the process steam supply occurs, this is tackled by installing a steam bypass-reducing de-superheating system with high pressure steam source. If there is any surplus steam produced by the system, this is vented to the atmosphere or bypassed to a dump condenser and the condensate returned to the power circuit. Herein is where energy wastage occurs and system efficiency is reduced.    b. In contrast to (a) above, there may be cases where the electrical generator is not operated standalone, i.e. the electrical generator is operated connected to a grid. Here the primary control will be to control the exhaust steam pressure. The process steam demand dictates the electrical power of the turbine generator and any surplus or deficiency in exhaust steam supply is made up by adjusting the power flow to the grid. This necessitates the turbine drifting away from the design operation point of maximum efficiency. Sometimes the process steam demand may drop drastically for short periods in which event excess steam will be vented to the atmosphere or bypassed to a dump condenser to keep the electrical power system stable or to maintain a minimum supply to the grid, but this again wastes energy reducing system efficiency.
To overcome the above shortcomings and to attain and maintain maximum efficiency in a CHP system all of the mechanical and thermal energies will need to be harnessed optimally.
The challenge in practice is to achieve efficient and economic operation irrespective of rapid and substantial changes in electrical and thermal loads. Variations in electric loads take place as various plant equipments are run up or shutdown. Simultaneously precipitous changes in thermal loads may take place as process heating plants are run up or shutdown and especially if batch processes are involved. These are common concerns among CHP plant operators that need to be addressed, necessitating the systems to be operated with precision controls, starting mechanisms, steam accumulators, and steam makeup bypasses systems to ensure the whole system functions at an optimum level. Venting off steam to the atmosphere to maintain stability is a common viability but this is a cause for a reduction in system efficiency and loss of pure water from the system. Similarly, throttling high-pressure steam to make-up for a lower pressure steam used in low-temperature heating is also an inefficient use of thermal energy.
On the demand side, heat and power demanded in a process industry varies rapidly and sporadically over a large range. Good concurrence between electric and thermal loads is desired in practice.
Efficient cogeneration systems are prized because they ensure high thermal efficiency for complete electric power and thermal load requirements, ensuring healthy bottom-lines. At design time, a CHP system is custom-tailored to strive for a balance between mechanical and thermal energy production to suit the needs of the end user plant. The system is usually configured with optimum design to cater for all operating conditions at the best economic point based on historical or predicted power and heat demand profile over a cycle.
It is imperative that cogeneration systems are operated at or near design operation point in order to ensure that overall thermal efficiency of the integrated system can be maintained at design high level. An adverse impact on the efficiency of the cogeneration system occurs when there is a change in the power-to-heat ratio which will then require an immediate response by way of either supplementary steam supply or power supply accordingly. Good concurrence between electric and thermal loads helps to limit energy losses. Operational flexibility in terms of the ability to adjust the system operation in quick response to changes in power or thermal energy demand without significantly sacrificing overall efficiency is much desired.
Where there is rapid and erratic variation of the process heat demand and this is left uncontrolled this could lead to consequential effects that ripple through to the boiler operation. Erratic load on the boiler causes undue stress that may reduce its useful life span. A steady state operation of the CHP plant is desired to not only extend life span of the boiler but also to help to maintain high combustion efficiencies and reduce air pollutant emission wherever a boiler supplies the primary heat to the system. Commonly boilers having an excess capacity are installed to ensure peak demand is met well within the capacity of the fired boiler. By reducing steam demand fluctuations on the boiler, the boiler steam drum size can be reduced for a given production capacity. This is an important factor to consider as the boiler operating pressures increase, because the cost of constructing and installing steam drums increases more than proportionally with boiler operating pressure increase.
In prior art, in order to maintain the required power or heat during these load changes (power-to-heat ratio balancing), either high pressure steam from the boiler is used to make up any deficiency in the process steam requirement or surplus steam is vented to the atmosphere or bypassed to a dump condenser and the condensate is returned to the power circuit. These are causes of potential thermal energy loss and corresponding fluid loses from the system. Further, dump condensers need a cooling source which can be a once-through circulating water or chilled water from a cooling tower, both of which require additional water source(s) and auxiliary power to keep them in standby operation.
It is therefore desired to seek an alternative solution that is both energy-efficient and water-saving and yet at the same time offers greater operational reliability and flexibility.